Update utils.dart to work with the breaking change of SDK issue 40678 (#53093)

dev/devicelab/lib/framework/utils.dart

@@ -19,11 +19,25 @@ import 'framework.dart';
 String cwd = Directory.current.path;
 
 /// The local engine to use for [flutter] and [evalFlutter], if any.
-String get localEngine => const String.fromEnvironment('localEngine');
+String get localEngine {
+  // Use two distinct `defaultValue`s to determine whether a 'localEngine'
+  // declaration exists in the environment.
+  const bool isDefined =
+      String.fromEnvironment('localEngine', defaultValue: 'a') ==
+          String.fromEnvironment('localEngine', defaultValue: 'b');
+  return isDefined ? const String.fromEnvironment('localEngine') : null;
+}
 
 /// The local engine source path to use if a local engine is used for [flutter]
 /// and [evalFlutter].
-String get localEngineSrcPath => const String.fromEnvironment('localEngineSrcPath');
+String get localEngineSrcPath {
+  // Use two distinct `defaultValue`s to determine whether a
+  // 'localEngineSrcPath' declaration exists in the environment.
+  const bool isDefined =
+      String.fromEnvironment('localEngineSrcPath', defaultValue: 'a') ==
+          String.fromEnvironment('localEngineSrcPath', defaultValue: 'b');
+  return isDefined ? const String.fromEnvironment('localEngineSrcPath') : null;
+}
 
 List<ProcessInfo> _runningProcesses = <ProcessInfo>[];
 ProcessManager _processManager = const LocalProcessManager();

dev/devicelab/test/utils_test.dart

@@ -33,4 +33,11 @@ void main() {
       expect(parseServicePort(badOutput), null);
     });
   });
+
+  group('engine environment declarations', () {
+    test('localEngine', () {
+      expect(localEngine, null);
+      expect(localEngineSrcPath, null);
+    });
+  });
 }